Synaptotagmins (SYTs) are transmembrane proteins involved in membrane trafficking and calcium-dependent exocytosis of synaptic vesicles at the synapse. SYTs may mediate this by binding to presynaptic proteins, the neurexins (NRXNs) (Perin 1994, Hata et al. 1993, Petrenko et al. 1991). The interaction between these two proteins may mediate part of the recognition of presynaptic active sites by synaptic vesicles or may regulate neurotransmitter release (Perin 1996).